Update seeds

Summary: Last time, we included both 0.19.x and 0.20.x nodes, but this time we have enough 0.20.x nodes to start serving only those by default.

Test Plan:
make

TOTAL=$(cat nodes_main.txt | wc -l); NUM=$(./test-seeds.sh < nodes_main.txt | wc -l); echo "scale=3; $NUM / $TOTAL" | bc
96.0% connectivity from my local machine

TOTAL=$(cat nodes_test.txt | wc -l); NUM=$(./test-seeds.sh --testnet < nodes_test.txt | wc -l); echo "scale=3; $NUM / $TOTAL" | bc
92.8% connectivity from my local machine

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4294